A subset of the nonnegative integers is called supported if it contains 0, and for all . How many supported sets are there?
Solution
Note that every supported set contains , 48-54, and all . Now define , which is a subset of satisfying the opposite property that . Consider the above arrangement after removing the numbers not in . The condition that be supported ensures that sets are in bijective correspondence with paths from to consisting of discrete steps of and and lying above the -axis: from the modified version of the above diagram, a unique path passes through the top items left in each column. The number of such paths is the 8th Catalan number, so the answer is . (Incidentally, 16 choose 8 was computed in an earlier problem.) Without the explicit formula for Catalan numbers, the answer can be computed recursively by filling in the number of ways a path can reach from each position in the figure. One works right to left, obtaining the following: One can exploit symmetry and, having determined the middle column, sum the squares: